The present invention relates to a device for the indirect radiant heating of running products, by means of the combustion of gaseous or liquid fossil fuels.
The present invention is intended more particularly for heat treatment furnaces in which the products (such as bars, tubes, strips and parts, for example) are heated on the move, in an environment which preferably must be free of certain gaseous components that result from the combustion. The invention may, for example, apply to the heating of steel strip running through heat treatment furnaces heated by fossil fuel and in which the strip is kept under a protective atmosphere.
According to the current state of the art, the indirect heating by fossil fuel of running products in a protective environment is generally performed using radiant tubes. FIG. 1 of the appended drawings diagrammatically depicts, in section on a vertical plane, one embodiment of so-called thimble-type radiant tubes which are used in installations with a low heat capacity. The tube, of cylindrical shape, is positioned between the walls of a furnace of width L. Mounted in the internal cavity of the tube is a concentric combustion tube inside which a burner is positioned and upstream of which a heat recuperator is mounted. Fuel is injected in the direction of the arrow G, oxidizer is injected in the direction of the arrow A and the flue gases are removed in the direction of the arrow F.
This exemplary embodiment of radiant tubes is characterized by a small volume in which combustion takes place, and this has the drawback of forming nitrogen oxides (NOx) in large quantity and of placing significant thermal stress on the combustion tube because of the high variation in temperature along this tube.
In industrial installations demanding the installation of significant heating power, this solution cannot be envisaged given that the number of thimble-type radiant tubes and of their associated burners needed would be very high and their cost excessive.
This is why, for high-capacity installations, use is made of multiple-strand (2-strand or 4-strand) radiant tubes. FIG. 2 of the appended drawings diagrammatically depicts one exemplary embodiment of a four-stranded radiant tube also known as a xe2x80x9cW tubexe2x80x9d, this tube, of circular cross section, being positioned between the walls of the furnace of width L. Fuel is injected in the direction of the arrow G, oxidizer is injected in the direction of the arrow A and the flue gases are removed in the direction of the arrow F.
The burner is mounted in the radiant tube, on the entry strand thereof and the heat recuperator is positioned on the strand via which the products of combustion exit. Combustion takes place in a cross section and in a volume which are larger than were the case in a thimble-type radiant tube illustrated in FIG. 1, and the absence of the internal combustion tube makes the device less expensive for the same overall thermal power. By contrast, the great length of such a radiant tube and the inevitable temperature differences between these various strands lead to a tendency to deformation generated by the thermal stresses and by the means supporting the intermediate strands. The systems for supporting such radiant tubes are complicated to produce and, in this respect, reference may be made to EP-A-0 383 687 which illustrates one embodiment of such support means.
The surface temperature differences of the tube along its entire length generally give rise to differences in heating across the width of the heated product, for example a strip, particularly between its center and its edges, and this may result in problems in controlling the tension across the width of such a strip.
In the particular case of a furnace for the vertical treatment of a strip as illustrated in FIG. 3, in which the tubes are placed between the strands of the strip, these radiant tubes according to the prior art have another drawback, namely a form factor which is not optimized for radiant heating because of the presence of gaps between the various tubes or between the various strands of the same tube.
The present invention proposes to provide a device for indirect radiant heating which does not have the drawbacks of the solutions of the prior art which have been examined hereinabove. This device has been designed to simultaneously solve the following various technical problems:
reducing the emissions of NOx;
improving the uniformity of the temperature of the surface radiating towards the products that are to be heated;
obtaining a form factor which is optimized for heating by radiating towards the products that are to be heated.
Consequently, this invention relates to a device for the indirect radiant heating, by means of the combustion of gaseous or liquid fossil fuels, of running products, such as bars, tubes, strips and parts for example, kept under a protective atmosphere, characterized in that it is produced in the form of a radiant cassette of essentially parallelepipedal shape, with a continuous radiating surface whose cross section, in a plane perpendicular to the axis of the cassette, is delimited by a continuous line which falls inside a rectangle whose height/width ratio is greater than 1.5. The radiant cassette includes a combustion tunnel equipped with a burner positioned inside the cassette, and the burner includes at least two fossil fuel injectors arranged parallel to the plane of the main face of the cassette so as to spread the flame parallel to the face. This produces a uniform distribution of the flame temperature parallel to the plane.
According to other embodiments of the invention, the cassette may be designed to operate according to the principle of so-called thimble-type radiant tubes or of U-shaped, W-shaped or E-shaped radiant tubes.
According to the invention, the device may be equipped with a heat recuperator on at least two faces of the cassette and over part of the length of these faces, it being possible for this recuperator to be of the tubular or finned type and it being possible for this recuperator to be used for heating the fuel or the oxidizer.
Other features and advantages of the present invention will become apparent from the description given hereinafter with reference to the appended drawings which illustrate various nonlimiting exemplary embodiments thereof.